The present application relates to an instrument for assembling a bone anchoring device, which includes a shank, a head being separate from the shank, and a receiving part having a first end, a second end and a longitudinal bore extending from the first to the second end, the head being positionable within the longitudinal bore at the second end.
Such a bone anchoring device is described, for example, in U.S. Pat. No. 6,835,196 B2, by the same applicant. The device is arranged to connect a screw with a rod. The shank of the screw and the head are formed as separate parts. The head is shaped as a spherical segment thus being adapted to be pivotably held in the receiving part for allowing a multiaxial arrangement of the device. One example disclosed relates to the shank which has a cylindrical shape while the separate head is provided with one or more slits and further with a cylindrical bore into which the cylindrical end portion of the shank can be inserted.
Another example of a bone anchoring device having a screw with a shank and a spherically segmented head, which is separate from the head, is disclosed in EP 1741396 A1, by the same applicant. The shank has a threaded section and an engagement end, which is of conical shape, wherein the diameter increases towards the free end. Similarly, the head has a bore which is of conical shape, wherein the respective cone angles may be the same.
The head according to EP '396 is also provided with one or more slits to enable an elastic widening of the diameter at the bottom opening of the bore, when the engagement end is inserted, whose end diameter is slightly larger than former diameter of the head. The widening is facilitated by a chamfer arranged at the edges of the engagement end. As a result of this configuration, the head may be clicked onto shank upon assembly wherein the conical shape allows for enhanced stability, once the head is clamped.
Another example of a bone anchoring device having a screw with a separate shank and head is disclosed in European Patent Application no. 06 024 232 not yet published. The spherically segmented head has a cylindrical shape similar to the example disclosed in U.S. '196 described above. However, for example, a ring opened by a slit is provided as an elastic projection positioned within a circumferential groove inside the bore of the head. The engagement end of the shank is also of cylindrical in shape and has substantially the same diameter as the bore of the head to be insertable therein. The engagement end has a corresponding circumferential groove that may receive the projection, which may thus snap into the groove upon insertion of the engagement end into the cylindrical bore of the head.
The separate provision of the shank and the head with respect to the screw makes it possible to assemble sets of bone anchoring devices, in which one head may be combined with shanks of various sizes, structure and material properties. This may reduce the costs of a set since the heads will generally have to satisfy strong requirements with regard to accuracy, stress and tension characteristics, selection of material, etc., while the shanks may be produced according to the specific needs in large number with slightly less accuracy. This becomes particularly valid in cases, where spherically segmented heads are used, which provide for multi- or polyaxial arrangements of bone anchoring devices.
Further, simple screwing-in of the bone screw into the bone may be accomplished followed by easily clicking the preassembled receiving part including the head onto the engagement end.
It has been observed, however, that the assembly of bone anchoring devices as described above may eventually become somewhat circumstantial, just when multi- or polyaxial arrangements are involved. For example, the bore of the head positioned inside the bottom opening of the receiving part may pivot in any direction away from the engagement end of the shank which can render the assembly as a difficult procedure.
Based on the foregoing, there is a need to improve and facilitate the assembly of bone anchoring devices, in particular where bone screws are provided which have separate shanks and heads.